1. Field of the Invention
This invention relates to an electrostatic protection circuit. In specific, this invention relates to an electrostatic protection circuit for protecting a photoelectric conversion circuit in a photoelectric conversion device including a thin film semiconductor. Moreover, this invention relates to a photoelectric conversion device including the electrostatic protection circuit and an electronic device including the photoelectric conversion device.
2. Description of the Related Art
A number of photoelectric conversion devices generally used for detecting an electromagnetic wave are known. For example, a photoelectric conversion device having sensitivity in ultra-violet rays to infrared rays are referred to as an optical sensor in general. Among them, an optical sensor having sensitivity in a visible light region with a wavelength of 400 to 700 nm is particularly referred to as a visible light sensor, and a large number of visible light sensors are used for devices which need illuminance adjustment, on/off control, or the like depending on human living environment.
In some display devices, ambient brightness of the display device is detected to adjust display luminance. This is because wasted electric power of the display device can be reduced by detecting ambient brightness with the use of an optical sensor and obtaining appropriate display luminance. For example, as display devices which include optical sensors for adjusting luminance, mobile phones and computers can be given.
In addition, as well as the ambient brightness of the display device, luminance of the backlight of a display device, in particular, a liquid crystal display device is also detected by an optical sensor to adjust the luminance of a display screen.
In an optical sensor, a photoelectric conversion element such as a photodiode is used for a light sensing part, and the output current of the photoelectric conversion element is amplified in an amplifier circuit. For example, a current mirror circuit is used as the amplifier circuit of the optical sensor (for example, see Reference 1: Japanese Patent No. 3444093)